Coal Utilization Technology Research Articles

Experimental study on devolatilization characteristics of lignite during pressurized oxy-fuel combustion: Effects of CO2 and H2O

Pressurized oxy-fuel combustion (POFC) is currently considered one of the advanced coal utilization technologies for large-scale CO2 capture but causes distinct coal conversion behavior, particularly more complicated and unclear with wet recycle. To explore coal conversion mechanisms and structural evolution of chars during the initial stage of POFC, lignite devolatilization was conducted in an isothermal horizontal tube furnace at 900 °C in individual and blending CO2 and H2O atmospheres (CO2/N2, H2O/N2, and H2O/CO2) under different pressures of 0.1 to 0.9 Mpa. The pore and chemical structures of resulting chars were characterized using N2 adsorption and Raman spectroscopy, and oxy-fuel combustion reactivities of chars were evaluated by thermogravimetry. The results indicate that coal char conversion in coexisting H2O/CO2 presents three different interactions of addition, synergy, and inhibition, corresponding to mesoporous surface area evolution of chars with partial and total pressures of H2O. CO2 chars primarily consist of micropores while chars formed in H2O-rich atmospheres contain more mesopores. These dominant micro-/mesopore numbers increase with CO2/H2O concentrations but decrease at 0.9 Mpa. The accelerated condensation of aromatic rings in chars induced by CO2 and H2O gasification also follows different pathways. H2O gasification dominates the carbon skeleton structure evolution of H2O/CO2 chars but the interaction between H2O/CO2 results in the complex variations of O-containing structures. CO2/N2 chars have the highest oxy-fuel combustion reactivities, follow by H2O/CO2 chars, and H2O/N2 char reactivities are the lowest. The combustion reactivities of H2O/CO2 chars show a more obvious attenuation trend than that of H2O/N2 chars and the disparities narrow with H2O concentration. Elevated total pressure decreases the reactivities of all chars along with the growth of aromatic ring sizes. The correlations between oxy-combustion reactivities and Raman band area ratios highlight the significance of the devolatilization stage to subsequent char combustion.

Importance evaluation for influencing factors of underground coal gasification through ex-situ experiment and analytic hierarchy process

Underground coal gasification is a promising technology for low carbon utilization of coal, while the importance of influencing factors for UCG performance is rarely evaluated quantitatively, which limits the optimization of UCG. In this work, the effects of flow rate, gasification agent, ignition position and initial channel size on temperature distribution and gas products were investigated through ex-situ UCG experiments. The effect of increasing flow rate has two sides, promoting oxidization and cooling down the reaction zone. Compared to air gasification, oxygen gasification features with a higher temperature and reaction zone is closer to upstream; at the same time, effective gas products are obviously more. Compared to ignition at 1/2 L0, the temperature is higher for ignition at 3/4 L0, and the effective gas production time (te) is shorter. When the initial gasification channel size increases from 1 cm to 2 cm, the reaction zone is closer to the upstream, and the te is longer. The analytic hierarchy process (AHP) method is adopted to compare the importance of these factors. Increasing oxygen concentration in the gasification agent is the most effective way to increase the maximum heating value, while increasing the flow rate is the most effective way to increase the te.

Semi-continuous Operation of Chemical Looping Combustion of Coal Using a Low-Cost Composite Oxygen Carrier

To reach the carbon peak by 2030 and achieve carbon neutrality by 2060, the implementation of low-carbon combustion approaches for fossil fuels (especially coal, a high-carbon fuel) is urgent in China. Chemical looping combustion (CLC) has been considered as one of the most promising technologies for low-carbon, efficient, and clean utilization of coal. However, a low-cost and high-performance oxygen carrier (OC) holds the bottleneck that constrains the industrial application of coal-fueled CLC. In this paper, a low-cost composite OC named Cu13.0Red87.0@C (with the mixing mass ratio of copper ore to red mud being 13.0:87.0, except for a 20 wt % cement bonder) was first prepared by the hydroforming method and then systematically evaluated by designing CLC tests in a semi-continuous fluidized bed using lignite as fuel. The results showed that this composite OC exhibited superior combustion performance in comparison to pure red mud. Moreover, it was found that increasing the temperature could significantly improve the average carbon capture efficiency but had a relatively small positive effect on the average CO2 yield. Additionally, with the elevation of the oxygen/fuel ratio, the resulting average carbon capture efficiency and average CO2 yield both tended to increase. Generally, the semi-continuous unit can simulate the continuously operated fuel reactor with an adjustable bed inventory and solid circulation rate, and it is easy to attain smooth operation of ca. 30 min, achieving both carbon capture efficiency and a CO2 yield of ca. 90%.

Analysis of the Characteristics of CH4 Emissions in China’s Coal Mining Industry and Research on Emission Reduction Measures

CH4 is the second-largest greenhouse gas and has a significant impact on global warming. China has the largest amount of anthropogenic coal mine methane (CMM) emissions in the world, with coal mining emissions (or gas emissions) accounting for 90% of total energy industry emissions. The results of CH4 emission inventories from previous studies vary widely, with differences in the spatial and temporal dimensions of gas emission factors of belowground mining being the main points of disagreement. Affected by the policies of “eliminating backward production capacity” and “transferring energy base to the northwest”, China’s coal production layout has changed greatly in the past ten years, but the closely related CH4 emission factors have not been dynamically adjusted. This paper investigated 23 major coal producing provinces in China, obtained CH4 emission data from coal mining, calculated CH4 emission factors in line with current production conditions, and studied the reduction measures of coal mine gas emission. According to the CH4 emission data of China’s coal mines in 2018, 15.8 Tg of methane is released per year in the coal mining industry in China, and 11.8 Tg after deducting recycling. Shanxi Province’s CH4 emissions are much higher than those of other provinces, accounting for 35.5% of the country’s total emissions. The weighted CH4 emission factor of coal mining in China is 6.77 m3/t, of which Chongqing is the highest at approximately 60.9 m3/t. Compared with the predicted value of the IPCC, the growth trend of CCM has slowed significantly, and the CH4 utilization rate has gradually increased. This change may be aided by China’s coal industry’s policy to resolve excess capacity by closing many high-gas and gas outburst coal mines. In addition, the improvement of coal mine gas extraction and utilization technology has also produced a relatively significant effect. This paper determines the distribution of methane emissions and emission sources in China’s coal mining industry, which is useful in formulating CCM emission reduction targets and adopting more efficient measures.

Study on supercritical water gasification reaction and kinetic of coal model compounds

Supercritical water gasification (SCWG) of coal is a clean coal utilization technology that has gradually developed in recent years. It has great application prospect because of low pollution, good economy and high energy efficiency. Due to the complexity of coal molecular structure, it is difficult to directly study SCWG mechanism of coal at molecular level. Based on the Wiser molecular structure model of coal, diphenyl ether and diphenylmethane were selected as model compounds to study the bond breaking characteristics and gasification law of characteristic bridge bonds. The effects of catalyst, temperature, concentration and other factors on model compounds gasification were discussed. The results showed that temperature had the most significant influence on gasification effect. The effect order of catalyst on promoting gasification from high to low was: K2CO3 > Na2CO3 > Ca(OH)2. The increase of catalyst amount and residence time had a certain promotion effect on gasification reaction. Under the same condition, the gasification effect of diphenyl ether was better than that of diphenylmethane. At the same time, the kinetic model of coal-like structure model compound gasification in SCW was established, and the key parameters were solved. The activation energy of diphenyl ether and diphenylmethane was 82.96 kJ/mol and 99.92 kJ/mol, respectively.

低炭素～脱炭素に向けた石炭利用技術の展望

低炭素～脱炭素に向けた石炭利用技術の展望

Current Situation and Foresight on Development of Coal Utilization Technology

Current Situation and Foresight on Development of Coal Utilization Technology

Thermodynamic analysis of the superiority of the direct mass transfer design in the supercritical water gasification system

Supercritical water gasification (SCWG) is a promising clean technology for coal utilization due to high chemical reactivity and no tar emission. However, the wall-type heat exchange in the SCWG system demands a large heating surface area and an extreme high-temperature heat source. This paper puts forward an optimized design, namely direct mass transfer, in which the final product gas is partially recycled back into the gasification reactor to supplement the gasification agent and provide sensible heat energy. A whole system simulation was established to analyze the mass flow and exergy efficiency in the reference design and the optimized design. The process analysis shows that the optimized design has higher exergy efficiency (85.6%) than that (72.5%) of the reference design. In the optimized system, direct mass transfer significantly alters the mass flow and exergy flow, with less influent water (33.3%), less heat transfer rate (30.5%), and less oxygen consumption (51.5%). Furthermore, sensitivity analysis shows that increasing circulation ratio, feedstock concentration, and preheating temperature can reduce oxygen consumption and improve gas output and efficiency. The study indicates that direct mass transfer has the potential superiority to solve the challenge of heat integration and facilitate the application of the industrial-scale SCWG system.

Effects of pressure on the chemical looping combustion of coal with CuFe2O4 combined oxygen carrier

Chemical looping combustion (CLC) has fast developed as an attractive technology for clean utilization of coal as well as inherent CO2 separation at the low energy penalty. Increase of the system pressure has been acknowledged as one of the effective measures in CLC for sufficient conversion of coal along with affiliated benefits for CO2 sequestration, but the complex consequences as incurred should be well considered. In this research, reaction of a typical Chinese bituminous coal of high sulfur content (designated as LZ) with CuFe2O4 combined oxygen carrier (OC) within 0.1–3.0 MPa in the CO2 atmosphere was systematically investigated, with focus on the effect of the system pressure to the conversion of LZ coal, transfer of the oxygen involved in CuFe2O4, sulfur evolution and redistribution. Sequential reactions of CuFe2O4 with the volatiles emitted from pyrolysis of LZ coal and further reaction with the released gasification products and residual char were accomplished, which was beneficial to overcome the mass transfer limitation of the reducing gases evolved from LZ coal to OC under the pressurized conditions and alleviate the inhibition effect resulting from accumulation of the transferred reactive gases around the surroundings of OC. And thus, under the pressurized conditions, improved conversion of LZ coal was reached by oxidization of more intractable C-C/C-H groups to oxygen bound carbon groups. Meanwhile, deep reduction of the added CuFe2O4 occurred with deficient Fe3O4 at the lower valence being formed, which would interact with the silicon minerals present in LZ coal to form various iron silicates. Furthermore, sulfur evolution from coal was found to directionally interact with the reduced CuFe2O4 to mainly form solid Cu2S due to their high combination potential. And deeper reduction of Cu2S was instigated especially at 3.0 MPa with Cu1.8S formed, and the total solid sulfur fraction was increased with the elevated system pressure. Overall, comprehensive information on the interaction of coal of high sulfur content with CuFe2O4 under the extended pressurized conditions around 0.1–3.0 MPa was provided, which would be meaningful for the realistic pressurized CLC in the future.

Long-term coal chemical looping gasification using a bimetallic oxygen carrier of natural hematite and copper ore

Coal chemical looping gasification (CCLG) is a promising gasification technology for coal utilization with high resource utilization rate, energy saving and environmental protection properties. Development of suitable oxygen carriers and optimization of operating conditions for CCLG are the key for the coal conversion and H2&CO production (H2, CO). In this study, Cu-Fe bi-ore oxygen carrier prepared by spray drying granulation method using hematite and copper ore as raw materials was tested for CCLG. The effects of the reaction temperature, oxygen carrier to coal (O/L) ratio and steam addition amounts were systematically evaluated by reaction tests combing with various analytical methods. Most importantly, over 100 redox chemical looping cycles were performed to demonstrate the excellent stability of Cu-Fe bi-ore oxygen carrier during the redox reactions. Results showed that the optimal gasification condition is 950 °C with oxygen carrier-to-coal mass ratio of 3:1 and steam rate of 0.08 ml/min. Syngas yield of 58 mmol/g, as well as 75.2% syngas selectivity and 85.3% carbon conversion were achieved. After 100 redox cycles, the synthesis gas yield can still reach 46 mmol/g despite a decline, and the carbon conversion kept above 72%, without obvious sintering in the long-term test. The characterization results showed that the synergistic effect between Cu and Fe metals contributed to its good oxygen release ability and reducibility. The Cu-Fe bi-ore oxygen carrier undergoes a gradual reduction Fe3+→Fe8/3+→Fe2+→Fe and Cu2+→Cu process during the gasification reaction, accompanying with the lattice oxygen migration from bulk to the particle surface. This work demonstrated the economical method for the industrial application of CCLG process using spray-drying-derived oxygen carriers.

Variation of pore structure in Zhundong coal particle with stepped K2CO3 loading during supercritical water gasification

Supercritical water gasification (SCWG) is a promising technology for coal utilization and has drawn lots of attention. K2CO3 is an outstanding catalyst for coal gasification and can significantly improve pore structure. The evolution of pore structure in coal particles affects heat and mass transfer efficiency, but little research has investigated the effect of K2CO3 on gasification results from the perspective of pore structure. Hence, this study revealed the variation of pore structure of coal chars with stepped K2CO3 loading during SCWG of coal by conducting experiment in a high-capacity autoclave reactor. Gas products and coal chars were characterized by GC, SEM, and BET to obtain the gasification result, surface morphology, and pore structure characteristics of coal chars. The results show that K2CO3 improved gasification efficiency, gas percentage, and gas yield of H2 and CO2. The time when K2CO3 changes pore structure is during the gasification process rather than before gasification. Macropores are dominant in coal chars. 0 wt%-8 wt% K2CO3 favors the formation of mesopores and macropores, while 10 wt% K2CO3 blocks the pore structure. 0 wt%-4 wt% K2CO3 loading makes a pore structure of the ink-bottle type, and 6 wt%-10 wt% K2CO3 loading makes the narrow-slit pore structure. When K2CO3 increases from 0 wt% to 8 wt%, production rate of C-containing gases has a good linear relationship with specific surface area, and pore volume also has a good linear relationship with carbon gasification efficiency. 10 wt% K2CO3 destroys the linear relationship by the blocking effect.

Effect of physicochemical properties of coal gasification fine ash on its wettability

Coal gasification, recognized as one of the most effective coal utilization technologies, will produce a certain amount of fine ash during the high-temperature reaction process. The wettability of gasification fine ash is a critical parameter to characterize the degree of high-temperature reaction and determine the separation efficiency of purification process. In the present work, the effect of physicochemical properties on the wetting behavior of different gasification fine ash is studied. The difference in wettability between particle size and types can be explained by the unique properties of particles (i.e. pore structure, mineral composition, and chemical structure). The results show that with the decrease in particle size, the surface morphology transforms from an irregular shape with a rough surface to a spherical shape with a smooth surface. The pore structure, characterized by fractal surface dimension Ds, presents positive correlations with the contact angle for a single type of fine ash. The mineral composition can just qualitatively assess the hydrophilicity of particles. Only the ratio of hydrophilic chemical structure can be used as a generic parameter to describe the wetting performance. Meanwhile, the wettability of hydrophilic particles can be enhanced by increasing moisture content, but there is no significant effect of moisture content on the wettability of hydrophobic particles.

Mechanism analysis and simulation of methyl methacrylate production coupled chemical looping gasification system

Nowadays, the efficient and cleaner utilization of coal have attracted wide attention due to the rich coal and rare oil/gas resources structure in China. Coal chemical looping gasification (CCLG) is a promising coal utilization technology to achieve energy conservation and emission reduction targets for highly pure synthesis gas. As a downstream product of synthesis gas, methyl methacrylate (MMA), is widely used as raw material for synthesizing polymethyl methacrylate and resin products with excellent properties. So this paper proposes a novel system integrating MMA production and CCLG (CCLG-MMA) processes aiming at “energy saving and low emission”, in which the synthesis gas produced by CCLG and purified by dry methane reforming (DMR) reaction and Rectisol process reacts with ethylene for synthesizing MMA. Firstly, the reaction mechanism of CCLG is investigated by using Reactive force field (ReaxFF) MD simulation based on atomic models of char and oxygen carrier (Fe2O3) for obtaining optimum reaction temperature of fuel reactor (FR). Secondly, the steady-state simulation of CCLG-MMA system is carried out to verify the feasibility of MMA production. The amount of CO2 emitted by CCLG process and DMR reaction is 0.0028 (kg CO2)−1·(kg MMA)−1. The total energy consumption of the CCLG-MMA system is 45521 kJ·(kg MMA)−1, among which the consumption of MMA production part is 25293 kJ·(kg MMA)−1. The results show that the CCLG-MMA system meets CO2 emission standard and has lower energy consumption compared to conventional MMA production process. Finally, one control scheme is designed to verify the stability of CCLG-MMA system. The CCLG-MMA integration strategy aims to obtain highly pure MMA from multi-scale simulation perspectives, so this is an optimal design regarding all factors influencing cleaner MMA production.

The multi-scale pore structure of superfine pulverized coal. Part 1. Macropore morphology

The accurate description of the complex multi-scale pore structure plays a crucial role in efficient coal utilization. However, there are few studies about the macroporosity in pulverized coals, which is important for understanding the migration paths and mechanisms. In this work, both macropore geometry and topology of superfine pulverized coal were characterized through mercury intrusion porosimetry (MIP). Furthermore, the permeability of coal with different sizes was addressed based on the percolation theory, and the compressibility was discussed. Finally, the macropore morphology was comprehensively evaluated based on three different fractal models. The results show the pore geometry in superfine pulverized coal can be depicted as the isolated large pore clusters occurring in continuous small pore networks. With the decline in particle size, the compressibility drops by 2.7 × 10−11 Pa−1 and 1.2 × 10−11 Pa−1 for NMG and HN samples. In addition, the fractal analyses confirm that with decreasing particle sizes, the surface fractal dimensions increase for both NMG and HN, whilst the structure fractal dimensions show the opposite. The pore geometry and topology have a combined effect on the permeability, which is correlated to both surface and structure fractal dimension, leading to the maximum permeability around 20 μm for studied coals (954.87 mD for NMG_18.7 and 482.08 mD for HN_23.9). The macropore morphology obtained here is useful for constructing a more comprehensive coal pore network, and better understanding the mass and heat transfer in coal particles, which sheds lights on further exploring the reaction mechanisms of new clean coal utilization technologies.

Multi-scale modeling and control of chemical looping gasification coupled coal pyrolysis system for cleaner production of synthesis gas

The cleaner and efficient utilization of coal is of great significance to achieve energy conservation enhancement and CO2 emission reduction targets. Coal pyrolysis (CP) and chemical looping gasification (CLG) are both new coal utilization technologies. A cleaner system integrating CP and CLG processes is put forward in this paper to produce high purity synthesis gas with lower CO2 emission for the sustainable development of coal industry, with the aid of multi-scale modeling means including molecular dynamics (MD) simulation, computational fluid dynamics (CFD) simulation, and process simulation. First, MD simulation is executed for CP process with a coal model consisting of 3860 atoms to obtain the optimum operation parameters by mechanism analysis. The obtained coal density of 1.2 ± 0.11 g/cm3 is consistent with the experimental result of 1.28–1.33 g/cm3, ensuring the optimized coal geometry model below 1100 K. Second, the CLG process is simulated by CFD method. CuO is selected as oxygen carrier based on its better heat transfer performance before its residence time is determined based on the fuel reactor (FR) model. Then the CP-CLG system is simulated with Aspen Plus software on the basis of above MD and CFD results. The process simulation result indicates that the purity of synthesis gas produced by CP-CLG system is about 20% higher than single coal CLG system as the latter generates much more CO2. Finally, one control scheme is designed to keep the operational stability of CP-CLG based cleaner synthesis gas production system with the aid of Aspen Dynamics software.

Modeling, prediction and multi-objective optimization of the coal gasification system

As global energy demand continues to increase, coal as basic energy still accounts for a significant proportion. Under the pressure of environmental protection, clean and efficient coal utilization technologies are in great demand. Coal gasification technology has the potential to realize near-zero-emissions for coal utilization. This paper establishes the coal gasification system model and analyzes the effect of oxygen/coal ratio and water/coal ratio on the system performance index of cold syngas efficiency, effective component ratio, carbon conversion ratio, and production ratio of hydrogen. The results show that when the oxygen/coal ratio increases, the efficiency of cold syngas and effective components ratio increase first and then decrease, carbon conversion ratio first increases and then remains unchanged, hydrogen production ratio gradually decreases; When the steam/coal ratio increases, the cold syngas efficiency, and carbon conversion ratio first increase and then decrease, effective component ratio ingredients gradually decreases, and the hydrogen production ratio increases. Using BP neural network to realize the prediction of the gasification system, and the mean square error reaches the magnitude of 10e-7. Multi-objective optimization results show that the oxygen/coal ratio and steam/coal ratio corresponding to the highest production ratio of hydrogen is 0.52 and 0.05. The highest carbon conversion ratio corresponds to the oxygen/coal ratio of 0.95 and the steam/coal ratio of 0.05.

Unsteady Heat Conduction Study During Underground Coal Gasiflcation For Constructing an In-situ Pyrolysis Project.(Dept.M)

Underground coal gasification (UCG) Is a potential coal utilization technology, receiving renewed interest around the world. In the last .five years there has been Increasing Interest In tile world energy and mining Industry for tile development of a new operating underground coal gasification systems to generate synthetic gas from underground coal seams. In this paper, the problem of unsteady heat conduction In the underground coal and rock layers during underground gasification Is considered. The heating of coal results In releasing considerable amount of gases mainly hydrogen and methane which would be collected and pumped through pipelines for use in different Industrial applications. In the present analysis, a spherical type heat source having a surface temperature of (1000OC, 1500OC, 1750OC and 2000OC) is used for heating the coal layer for achieving tile pyrolysis process. The differential equations describing the unsteady heat conduction in both coal and rock layers are solved numerically using the discretization method (control volume formulation). A computer program Is constructed for calculating the temperature fields and the gasified volumes of mineral coal as well as the heating velocity in the coal seams. In the experimental part, a model of an underground coal layer Is constructed. To validate the numerical model, an electrical spherical heat source Is Inserted directly into the coal layer for heating. The temperature distribution In coal layer is measured In different locations using the Thermocouples. The results show that the temperature profiles and the maximum attainable temperature in the coal seams depend on the time and the surface temperature of the heating reactor. Furthermore, the gasified volume of coal increase with the increase of both the surface temperature of the heating source and the running time. It is found also that the heating velocity in the underground coal. Lies between the mean value (2 to 10-3 K/min.) and increase with the time to reach a maximum value, after this it decrease to approach zero at the steady-state conditions. This study indicates that geometry, position, surface temperature and distribution of heating sources are very important parameters which should be carefully chosen for realizing an efficient pyrolysis of underground coal and avoiding thereby the combustion of a part of the coal seams. The analysis indicated that the best temperature is between (700-800oC) at which the high efficiency of pyrolysis process, the analysis might be useful in constructing an in-situ pyrolysis project for utilizing the underground coal seams.

Study on the pyrolysis of different coal in atmospheres of argon, carbon dioxide, and steam

The pyrolysis process is fundamental in many deep coal utilization technologies. The comparison of conversion of three different types of coal was carried out utilizing thermal analysis in the atmosphere of argon, carbon dioxide, and steam within the temperature range from ambient temperature to 1000°C. The anthracite, bituminous coal, and lignite of Siberian deposits were studied. It was found that within the temperature range of 20-700°C for bituminous coal and lignite, and within 20-800°C for anthracite, the differences between mass loss rate in inert medium and other studied samples were insignificant. While flue gases (from fossil fuels burnt with low excess air ratio) in general consist of carbon dioxide and steam. Thus, they could be used for the pyrolysis of studied fuels within the mentioned temperature ranges.

Energy and exergy analyses of S–CO2 coal-fired power plant with reheating processes

S–CO2 (Supercritical-CO2) coal-fired power plant is a promising technology for efficient and clean utilization of coal for power generation. The conversion and transfer of the energy and exergy in the power plants with double-reheat and single-reheat processes are studied. With the main gas parameters of 32 MPa/893.15 K, the power generation efficiencies of the S–CO2 coal-fired power plant with double-reheat and single-reheat processes are 49.06% and 48.72%, respectively. The corresponding exergy efficiencies are 48.02% and 47.69%, respectively. The origins of exergy destructions in different units are studied using the Energy Utilization Diagram (EUD) method. The exergy distributions of the power plants are presented. For the power plant with double-reheat process, the work output, the exergy exhaust into the atmosphere, the exergy destruction in combustion process, the exergy destruction in heat transfer processes, the exergy destruction caused by pressure loss, and the exergy destructions in turbo systems account for 48.02%, 9.66%, 20.45%, 17.56%, 1.08%, and 3.23% of the total exergy input of the power plant, respectively.

Release and transformation mechanisms of hazardous trace elements in the ash and slag during underground coal gasification

Underground coal gasification (UCG) is a new coal utilization technology with advantages of high safety and high efficiency. However, the risk of hazardous trace elements (HTEs) in the UCG residue to the underground environment has always been an important issue. In this paper, the UCG sequential transformation products were prepared, and the chemical speciation of hazardous trace elements (Hg, As, Se, Pb, Cr, and Cd) in the UCG products were studied by using a combination of chemical analysis and thermodynamic calculation. Moreover, the risk assessment code (RAC) was introduced to evaluate the risk level of hazardous trace elements to underground environment. The results indicate that during the UCG pyrolysis process, Hg, Pb, Se, and Cd in the Ulanqab coal react with S to form HgS, PbS, SeS, and CdS, respectively. The oxidizable form of Hg (50.00 ± 0.18–66.67 ± 0.23%) and As (78.34 ± 0.37–83.36 ± 0.46%) is the main speciation in 900–1300 °C reduction ash. For the oxidation ash and slag samples, Hg is loaded on the mineral system of Na-Si-O in the form of HgO, and As is combined with the FeAl2O4. Based on the risk assessment code (RAC), Hg, Se and Pb have medium risk level in the coal and semi-coke. Cr and Se in the 900–1200 °C reduction ash can pose high risk on the underground environment. In the oxidation ash and slag, the risk level of HTEs to the environment decreases. The results can provide scientific guidance for the control and treatment of residues in the later-stage of underground coal gasification.